The present invention relates generally to fiber optic interface modules and associated fabrication methods and, more particularly, to fiber optic interface modules, including both fiber optic transmitter modules and fiber optic receiver modules, that are designed to maintain the high performance requirements demanded by avionics and other specialized applications in a more cost effective manner, as well as an associated fabrication method.
Fiber Data Distributed Interface (FDDI) modules are used in a variety of applications. For example, modern commercial aircraft, such as the Boeing 777 aircraft, includes a number of FDDI modules, including both fiber optic receiver modules, fiber optic transmitter modules and fiber optic transceiver modules. In addition, FDDI modules are also utilized by a variety of less demanding commercial applications.
Typically, an FDDI module serves as a data link between stations of a local area network (LAN). In basic terms, an FDDI fiber optic receiver receives optical signals such as optical signals delivered by an optical fiber, and converts the optical signals into corresponding electrical signals that can then be processed or analyzed as desired. Conversely, an FDDI fiber optic transmitter receives electrical signals and converts the electrical signals into corresponding optical signals that are then typically launched through an optical fiber.
The fiber optic modules that are employed for avionics applications must meet high performance standards over a wide range of temperature and environmental conditions. In avionics applications, for example, the fiber optic transmitters must produce an output pulse that has well defined rising and falling edges and that has a shape as defined by a predetermined envelope over a large range of temperatures and other environmental conditions. The fiber optic receivers must also exhibit high performance characteristics, such as high sensitivity and high dynamic range and, in some instances, may have a sensitivity better than xe2x88x9234 dBm, over a wide range of temperatures and other environmental conditions. In this regard, the FDDI fiber optic modules must be capable of maintaining consistently high performance levels at any operating temperature from xe2x88x9255xc2x0 C. to 110xc2x0 C. and over a correspondingly wide range of humidity levels and other severe avionics environmental conditions. In addition, both the fiber optic transmitter modules and the fiber optic receiver modules must be capable of transmitting data at a high bit rate, such as at a rate of at least 125 megabits per second.
In order to meet the high performance requirements over the entire range of temperature and other environmental conditions, FDDI fiber optic modules that are designed for avionics and other specialized applications include a number of relatively expensive components, thereby driving up the overall cost of the FDDI fiber optic modules. For example, FDDI fiber optic modules that are designed for avionics applications typically include a ceramic substrate which is not only quite expensive, but which also demands relatively costly assembly procedures that further increase the cost of the resulting modules. The FDDI fiber optic modules that are designed for avionics applications also generally include a relatively expensive, gold plated, custom designed metal package which is capable of being hermetically sealed in order to protect the enclosed fiber optic module.
Additionally, avionics grade fiber optic receivers and transmitters typically include optoelectronic devices, such as light emitting diodes and the light sensitive detector diodes, that are custom made in order to fit within the package while providing the desired performance levels. The optoelectronic devices of conventional avionics grade fiber optic modules are expensive since, among other things, the optoelectronic devices generally are custom designed fiber pigtail packages which require active alignment of the optical fiber-to-the-optoelectronic device. In this regard, the optical fiber would have to be actively aligned to the light emitting diode of a fiber optic transmitter module or the light sensitive diode of a fiber optic receiver module. As will be apparent, a fiber pigtail package is relatively expensive since the preparation of the optical fiber and the alignment of the optical fiber are both time consuming and labor intensive. In addition, since the optoelectronic devices include attached pigtails, the optoelectronic devices must be more carefully handled during he manufacturing process in order to prevent the fiber pigtail from being broken.
The relatively high cost of the components of the FDDI fiber optic modules that are designed for avionics applications is exacerbated by the relatively low volume of the fiber optic modules. In this regard, the volume demand for avionics grade fiber optic modules generally ranges from a few modules per year to a thousand per year. This low volume stands in marked contrast to the fiber optic modules employed in other commercial applications that have volumes of many thousands per year. As a result of their relatively high cost, the avionics grade fiber optic modules therefore cannot effectively compete in the commercial market in an attempt to increase the sales volume and correspondingly decrease the price of the avionics grade fiber optic modules.
In an attempt to reduce the cost of avionics grade fiber optic modules, automatic alignment processes have been implemented to reduce the labor costs and time delays associated with the alignment of a fiber pigtail and an optoelectronic device. In addition, attempts have been made to increase the commonality of the components that form the fiber optic receiver modules and the fiber optic transmitter modules as well as to increase the commonality between the processes employed to fabricate the fiber optic receiver modules and the fiber optic transmitter modules. Unfortunately, these attempts have generally failed to significantly reduce the cost of the avionics grade fiber optic modules, primarily because the optoelectronic components remain quite expensive and since the fabrication process has neither a high yield nor a high throughput.
Accordingly, it would be desirable to provide lower cost FDDI modules, such as fiber optic receiver modules and fiber optic transmitter modules, that meet or exceed the high performance requirements demanded by avionics and other specialized applications over a wide range of temperatures and other environmental conditions. While conventional fiber optic interface modules that are designed for avionics and other specialized applications have excellent performance characteristics over a wide range of temperatures and other environmental conditions, these fiber optic interface modules remain quite expensive, thereby effectively limiting their use in other commercial applications that need not be designed to withstand such severe environmental conditions.
According to the present invention, more cost-effective fiber optic interface modules, such as fiber optic transmitter modules and fiber optic receiver modules, are provided that meet the high performance requirements demanded by avionics and other specialized applications over a wide range of temperatures and other environmental conditions. In this regard, the fiber optic interface module of the present invention includes a greater number of commercial components that are capable of being assembled according to newly developed commercial fabrication techniques such that the cost of the resulting fiber optic interface modules are significantly reduced relative to conventional avionics grade fiber optic interface modules, i.e., the fiber optic interface module of the present invention has a greater performance-to-cost ratio. Additionally, a method of fabricating a fiber optic interface module is provided according to another aspect of the present invention.
In one embodiment, a fiber optic transmitter module is provided that includes a printed wiring board defining a notch in at least one edge thereof. In this regard, the printed wiring board generally includes a pair of arm portions that extend outwardly and define the opposed edges of the notch. The printed wiring board typically includes a substrate and electrical traces defined thereupon. The fiber optic transmitter module also includes an integrated circuit mounted upon a medial portion of the printed wiring board such that the printed wiring board mechanically supports the integrated circuit. Among other components, the integrated circuit of the fiber optic transmitter module includes a driver circuit for providing the drive current.
The fiber optic transmitter module also includes a transmitter optical subassembly including a receptacle and a light source, such as a light emitting diode, that is responsive to the driver circuit for transmitting signals in response to the drive current. In one advantageous embodiment, the receptacle defines an internal cavity for receiving the light source and a pair of slots proximate the internal cavity. According to the present invention, the transmitter optical subassembly is mounted to the printed wiring board so as to be at least partially disposed in the notch defined by the printed wiring board. In this regard, the respective arm portions of the printed wiring board are preferably inserted into the pair of slots defined by the receptacle in order to permit a portion of the transmitter optical subassembly to be disposed within the notch. As such, the path length between the light source and the integrated circuit is effectively reduced, thereby correspondingly reducing circuit parasitics and improving the performance of the fiber optic transmitter module.
Both the light source and the integrated circuit are preferably electrically connected to the same electrical trace that has a path length selected such that the circuit parasitic capacitance and inductance are minimized, typically such that the total circuit parasitics are less than 2 picofarads. For example, both the light source and the integrated circuit are preferably electrically connected to the same impedance (e.g., 50 ohm) matched electrical trace that defines a linear path across the printed wiring board (PWB). In addition, the electrical leads by which the light source connects to the (50 ohm) impedance matched electrical trace on the printed wiring board preferably has a path length that is no greater than 2 millimeters. As a result of the short, straight electrical path, circuit parasitics are substantially reduced. Using the method of the current invention, the electrical leads from the light source are soldered directly on the (50 ohm) impedance matched electrical traces without wire bonding, therefore eliminating the parasitic inductance associated with the bond wires.
A fiber optic receiver module is also provided according to another embodiment of the present invention. The fiber optic receiver module also includes a printed wiring board defining a notch in at least one edge thereof. Typically, the printed wiring board includes a pair of outwardly extending arm portions that define the opposite edges of the notch. The fiber optic receiver module also includes an integrated circuit mounted upon a medial portion of the printed wiring board. Among other components, the integrated circuit includes an amplifier for amplifying signals received by the fiber optic receiver module.
The fiber optic receiver module also includes a receiver optical subassembly having a receptacle and a detector, such as a light sensitive diode. Typically, the receptacle defines an internal cavity for receiving the detector and a pair of slots proximate the internal cavity. Like the transmitter optical subassembly, the receiver optical subassembly is mounted to the printed wiring board so as to be at least partially disposed in the notch defined by the printed wiring board. In this regard, the receiver optical subassembly is mounted such that the pair of arm portions extend into the slots defined by the receptacle in order to permit portions of the receiver optical subassembly to be disposed within the notch. As such, the path length between the detector and the integrated circuit is reduced and the circuit parasitics are correspondingly reduced.
As also described above in conjunction with the fiber optic transmitter module, the fiber optic receiver module is preferably designed such that both the detector and the integrated circuit are electrically connected via an impedance matched (e.g., 50 ohm) electrical trace that has a relatively short, collective path length such that the circuit parasitic capacitance are minimized, typically such that the total circuit parasitics are no greater than 2 picofarads. In this regard, the (50 ohm) impedance matched electrical trace(s) to which the detector and the integrated circuit are electrically connected typically defines a linear path across the printed wiring board. In addition, the electrical leads by which the detector connects to the impedance (e.g., 50 ohm) matched electrical traces on the printed wiring board is no greater than 2 millimeters. By establishing an electrical connection along a relatively short, straight electrical path, the fiber optic receiver module also reduces the circuit parasitics. Using the method of the current invention, the electrical leads from the detector are soldered directly on the impedance (50 ohm) matched electrical traces without wire bonding, therefore eliminating the parasitic inductance associated with the bond wires.
According to these embodiments of the present invention, a fiber optic transmitter module and a fiber optic receiver module are provided that can be fabricated with more lower cost, commercial off the shelf components and assembly equipment, while still providing the high performance over the wide range of temperatures and other environmental conditions that is demanded for avionics and other highly specialized applications. In this regard, the printed wiring board is specifically designed to define a notch between a pair of arm portions such that the respective optical subassembly can be at least partially mounted within the notch in order to reduce the path length of the electrical trace that extends between the optical subassembly and the integrated circuit. As such, the circuit parasitics are correspondingly reduced, thereby permitting the fiber optic transmitter module or the fiber optic receiver module to operate at relatively high speeds and to demonstrate a relatively high performance level over a wide range of temperatures and other environmental conditions even though the cost of the fiber optic transmitter module or the fiber optic receiver module is significantly less than conventional avionics grade fiber optic interface modules. Accordingly, the fiber optic transmitter module and the fiber optic receiver module have a larger performance-to-cost ratio than conventional designs.
According to another aspect of the present invention, a method of fabricating a fiber optic interface module, such as a fiber optic transmitter module or a fiber optic receiver module, is provided. Initially, a printed wiring board, such as a laminated printed wiring board, is provided with a U-shaped notch defined in at least one edge thereof. In addition, the printed wiring board can include outwardly extending arm portions on opposite sides of the notch.
An optical subassembly is then mounted to the printed wiring board so as to be at least partially disposed in the notch defined by the printed wiring board. In order to position a portion of the optical subassembly in the notch, the arm portions of the printed wiring board are preferably inserted into slots defined by the receptacle of the optical subassembly. In order to further reduce the manufacturing costs of the fiber optic interface module, the optical subassembly is preferably mounted to the printed wiring board by surface mount technology. In this regard, the optical subassembly is surface mounted to the printed wiring board and the leads of the optical subassembly are soldered to establish electrical contact with the electrical traces disposed upon the printed wiring board. Alternatively, the optical subassembly can be mounted to the printed wiring board by means of plated through holes defined by the printed wiring board.
An integrated circuit is then mounted upon a medial portion of the printed wiring board. In instances in which the fiber optic interface module is a fiber optic transmitter module, the integrated circuit is a transmitter integrated circuit having a driver circuit for providing a drive current. Alternatively, in instances in which the fiber optic interface module is a fiber optic receiver module, the integrated circuit is a receiver integrated circuit-having an amplifier for amplifying signals that are received by the fiber optic interface module. Irrespective of the type of integrated circuit, however, the integrated circuit is preferably mounted to the printed wiring board by adhesively bonding the integrated circuit to the printed wiring board. In addition, electrical connection by wire bonds can be established between the integrated circuit and wire bonding pads disposed upon the printed wiring board.
Accordingly, the method of this aspect of the present invention permits fiber optic interface modules to be fabricated with lower cost components that are assembled by newly developed manufacturing techniques that are more cost effective, while still providing the high speed and desired performance over the wide range of temperatures and other environmental conditions demanded for avionics and other specialized applications. For example, the fiber optic transmitter and receiver modules and the fabrication method of the present invention typically include commercial-grade optical subassemblies that are mounted to laminated printed wiring board in order to reduce the cost of the resulting fiber optic interface module. In addition, surface mount technology and wire bonding techniques are preferably employed in order to reduce the fabrication costs. By specifically designing the printed wiring board to define a notch in which the optical subassembly is at least partially disposed, however, the path length of the electrical trace between the optical subassembly and the integrated circuit is reduced, relative to conventional designs, thereby correspondingly reducing the circuit parasitics. As such, the fiber optic interface module of the present invention can perform at least as well as the conventional avionics grade fiber optic interface modules over the wide range of temperatures and other environmental conditions demanded by avionics and other specialized applications even though the fiber optic interface modules of the present invention are much less expensive.